Yep, you should go. If he is under 18 years you may have to go. Some practitioners will require a parental presence.
I also think it's prudent for a parent to review the medical history questionnaire as well.

If it is required in that particular state, you/son will be told so when ther appointment is made. To be on the safe side, you/son can certainly ask.

If that is a state where parental accompaniment is not required, then it is a family decision. Both sides of the issue can be argued successfully. As Just a Mom said being one side AND this is a transition period, so letting them do it themselves can also be argued.

I think the compromise is the best...parent goes and is present in the waiting room. Son/daughter handles the exam on their own, but the parnet is readily available if there are questions. I saw this actual example when visiting one of our clinics in Louisville, KY.